(a) Field of the Invention
The present invention relates to a converter. More particularly, the present invention relates to a quasi-resonant converter.
(b) Description of the Related Art
A converter is a power device for converting an AC signal and a DC signal, and it is used in a switching mode power supply (SMPS). It includes an AC/DC converter which converts an AC signal into a DC signal, a DC/DC converter which converts a DC signal into another DC signal, and an inverter which converts a DC signal into an AC signal.
In general, the converter includes a transformer for receiving a DC voltage, and a main switch which is coupled to the primary coil of the transformer. The converter maintains the DC output voltage or current of an output unit by controlling a turn-on time of a main switch with a feedback loop to transmit a voltage or a current of the output unit through a photocoupler and a shunt regulator coupled to a secondary coil of the transformer.
A general quasi-resonant converter is driven to turn on a main switch when the voltage difference at both terminals of the main switch reaches the valley of a resonance waveform for the 1st time after the main switch is turned off, and it thereby minimizes power consumption caused by switching of the main switch.
However, the general quasi-resonant converter greatly increases the switching frequency of the main switch when the input voltage exceeds a predetermined level or a load at the output terminal of the converter becomes very small. Increased switching frequency resultantly increases power consumption caused by switching of the main switch.
Recently, in order to solve the problem, techniques for turning on the main switch when a voltage difference at both terminals of the main switch reaches the second valley or the third valley other than the first valley to avoid increasing switching frequency have been proposed.
However, the proposed techniques require a complicated circuit configuration to detect the time instant when the voltage difference reaches the valley of the resonance waveform, and hence, the production cost and layout area of the converter are increased.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.